Negative photoresist masks are commonly employed in lithographic processes used to build integrated circuit boards. As is known in the art, lithography is a process by which geometric shapes are transferred from a mask onto the surface of a substrate. These geometric shapes, when transferred onto the surface of a silicon wafer, make up the parts of the circuit, such as gate electrodes, contact windows, probe electrodes, switches, metal interconnections, and the like.
The steps of the lithographic process are generally as follows. First, a base semiconductor (e.g., silicon) substrate is provided, and a second material layer, such as an oxide layer, is formed thereon. Next, a photosensitive polymer film or photoresist layer is deposited on the wafer, evenly covering the second material layer. Then, the photoresist layer is selectively fixed to produce a desired pattern thereon. This is done by selectively exposing the photoresist layer with ultraviolet light or other radiation, with the selective exposure defining the desired pattern.
Then, the unfixed portions of the photoresist layer are removed by a solvent rinse. (The unfixed portions may be either the exposed or non-exposed areas of the photoresist layer depending upon the type of polymer used.) At this point the surface of the wafer is covered by a substrate and a second material layer, with a desired design “masked” thereon.
The next step is to etch away the exposed second material layer while leaving the fixed portions of the photoresist layer intact. This is typically done by etching with an etchant that attacks the second material layer but not the photoresist layer, such as by hydrofluoric acid (HF), etching the exposed silicon dioxide (SiO2) while leaving the HF-resistant polymer photoresist layer intact. Excess etchant is rinsed away after the etching step, followed by a chemical removal of the photoresist layer with a solvent solution. The wafer is then rinsed to remove excess solvent and is now ready for the next photoresist step to occur.
A typical IC device requires between about five and 30 lithographic deposition layers, more if the device is fairly complex. Each of those layers corresponds to a separate type of device in the circuit, such as gate electrodes, contact windows, probe electrodes, switches, metal interconnections, and the like.
It can be seen from the above that the photoresist masking material must adhere firmly to the substrate, and must maintain planarity to very high tolerance levels.
One group of commonly used photoresist materials are those based on octafunctional epoxidized novolac resins, and particularly those based on the SU-8 family of photoresists made from Shell Chemical's EPON® SU-8 epoxy resin. The SU-8 photoresist materials are negative, epoxy-type, near-UV photoresist materials. The SU-8 family of photoresist materials is popular because it can provide relatively thick-film (2 mm or larger) photoresist films having aspect ratios greater than 20 at relatively low costs. In addition to being a relatively thick-film photoresist material, SU-8 is well suited for acting as a mold for electroplating because of its relatively high thermal stability (Tg greater than 200 degrees C. for the cross-linked or post-exposure photoresist material).
While the SU-8 family of photoresist materials enjoys relatively wide use and positive results as a photoresist in IC lithography, its utility is somewhat limited for use with silicon wafers greater than three or four inches in diameter. As with most polymer photoresists, the SU-8 family of photoresist materials is prone to delamination and via cracking arising from high film stress when used with relatively large silicon wafers.
Another problem with the SU-8 family of photoresists arises from adhesion problems with certain substrate materials. For example, SU-8 does not adhere as readily to metallic copper as it does to copper oxide.
The IC device industry is constantly demanding larger and/or more complex devices. As the size and complexity of the demanded IC devices increase, the need arises for a photoresist material having reduced film stress and increased adhesion properties for use in the lithography process to make such devices. The present invention addresses that need.